Method for producing a pressurised air gas by means of cryogenic distillation

ABSTRACT

The invention relates to a method for separating air by means of cryogenic distillation in a system of columns, in which two single-stage air superchargers are connected in series and coupled to two turbines, which expand the air that was not supercharged. The superchargers supercharge the cooled high-pressure air in an exchange line in which the oxygen from the system of columns is vaporized.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a §371 of International PCT ApplicationPCT/FR2012/050701, filed Mar. 30, 2012, which claims the benefit ofFR1152734, filed Mar. 31, 2011, both of which are herein incorporated byreference in their entireties.

TECHNICAL FIELD OF THE INVENTION

This invention relates to a method and an apparatus for producing apressurised air gas by means of cryogenic distillation.

BACKGROUND

The industrial state of the art, for apparatuses producing pressurisedoxygen of about 15 bars, is comprised of “pump” apparatuses, which use amain air compressor at a pressure of about 6 bars, and an air boostercompressing a portion of the airflow at a pressure of about 35-40 bars.But this solution is not available for apparatuses of small size, forwhich the combination of a small flow to be boosted and a very highdischarge pressure leads to an actual flow at the outlet of the boosterthat is too small to be carried out technologically.

So, for small apparatuses, oxygen compressors have to be used, which areexpensive.

The solution proposed makes it possible to reduce the costs for suchapparatuses, through the use of a single air compressor, at a moderatelyhigh discharge pressure, which proposes a competitive advantage inrelation to the two preceding solutions: the use of a single compressorand avoiding an expensive oxygen compressor.

US-A-20050126221 describes a method for separating air according to thepreamble of claim 1. To produce pressurised oxygen, two boosters inseries compress the air at intermediate temperatures of the mainexchanger, with the intake temperature for the first booster beinghotter than the outlet temperature of the second booster. A cooling unitis used to lower the intake temperature of the second booster, which assuch increases the complexity of the method.

US-A-20060010912 describes a method for separating air wherein air at amedium pressure is boosted in two cold boosters in series.

The two boosters must not be coupled to a turbine, because the turbinesof the method only operate during a particular operation making itpossible to manufacture liquid. In nominal operation, the method is keptcold by adding cryogenic liquid.

All of the pressures are absolute pressures.

SUMMARY OF THE INVENTION

One purpose of the invention is to propose an alternative for creatingprocess schemes which make it possible to improve the installation costsof devices for separating air for a production of oxygen between 10 and16 bars, more preferably between 14 and 16 bars, therefore about 15bars.

According to certain embodiments of the invention, all of the air isbrought to high pressure (substantially higher than the pressure of themedium-pressure column) and purified at this pressure, then divided intoat least two portions. Only one fraction of the air, the fraction thatis liquefied later at the cold end of the main exchange line, undergoesa succession of cryogenic compressions in such a way as to bring thisflow to a pressure that is enough to allow for the vaporisation ofoxygen at the desired pressure. The rest of the air is expanded in atleast one turbine to the pressure of the medium-pressure column. Atleast a portion of the work generated by the expansion of the air isused for the cryogenic compression.

According to an object of the invention, a method is provided forseparating air by means of cryogenic distillation in an installationcomprising a system of columns, including a column operating at thehighest pressure called the medium pressure wherein:

all of the air is brought to a high pressure, at least 3 bars higherthan the medium pressure, purified at this pressure in a purificationunit and air is sent at the output temperature from the purificationunit to an exchange line;

all of the purified air is cooled in the exchange line and a portionconstituting between 10% to 35% of the purified air is boosted by meansof at least one first single-stage booster with an inlet at anintermediate temperature of the exchange line;

at least one portion of the boosted air in the first booster is cooledin the exchange line, boosted by means of at least one secondsingle-stage booster and with an inlet at a second intermediatetemperature of the exchange line and is sent back into the exchange linewhere it is cooled, then is liquefied, possibly at the cold end of theexchange line and is sent into the system of columns after expansion;

another portion of the high-pressure purified air, constituting possiblybetween 65% and 90% of the high-pressure purified air, is cooled in theexchange line then at least partially expanded in at least two turbineshaving one or more intake temperatures which is an intermediatetemperature or which are intermediate temperatures of the exchange lineand then sent to the system of columns in order to be separated;

the work generated by the expansion of the air is used at leastpartially for the cryogenic compression carried out by the first and/orthe second booster by coupling the first booster to one of the twoturbines and the second booster to the other of the two turbines;

liquid oxygen is vaporised in the exchange line characterised in thatall of the air purified in the purification unit is sent at the outputtemperature of the purification unit to an exchange line, the liquidoxygen that has been pressurised at a pressure less than or equal to 16bars, more preferably between 10 and 16 bars, is vaporised in theexchange line, an energy dissipating device is coupled to at least oneof the boosters, the first temperature differs from the secondtemperature by at most 10° C. and the first and second temperatures arebetween −145° C. and −165° C.

According to other optional characteristics:

the two turbines have equal or different intake temperatures,constituted by the third intermediate temperature and a fourthintermediate temperature of the exchange line;

-   -   the third temperature is lower than the first temperature.    -   the third temperature differs from the fourth temperature by at        most 20° C., or even by at most 10° C.;    -   the first temperature is higher than the second temperature;    -   the first temperature is lower than or equal to the second        temperature;    -   a portion of the energy generated by at least one of the        turbines is dissipated;    -   a portion of the energy is dissipated by means of a hydraulic        brake system connected to the turbine;    -   a portion of the air is liquefied at high pressure, preferably        in the exchange line;    -   the air from at least one of the turbines is sent to the column        operating at the highest pressure;    -   all of the air boosted in the first booster is sent to the        second booster;    -   all of the air purified in the purification unit is sent to the        exchange line at the output pressure of the purification unit;    -   the system comprises a double column for separating air        comprising a first column and a second column operating at a        lower pressure than the first, with the air expanded in the two        turbines being sent to the first column;    -   the first temperature is colder than the output temperature of        the second booster;    -   the output temperature or temperatures of the first and/or of        the second booster is/are between −110° C. and −150° C.;    -   the output temperature or temperatures of the first and/or of        the second booster is/are between −125° C. and −145° C.

According to another object of the invention, an apparatus is providedfor separating air by means of cryogenic distillation comprising asystem of columns, of which one column operating at the highest pressurecalled the medium pressure, a compressor for compressing all of the airat a high pressure, greater by at least 3 bars than the medium pressure,a purification unit connected to the compressor to purify all of thehigh-pressure air, a pipe for sending a portion constituting between 10%to 35% of the high-pressure purified air to be cooled in an exchangeline, a first single-stage booster, a second single-stage booster, apipe for sending the portion constituting between 10 and 35% of the airto be purified in the first booster at a first intermediate temperatureof the exchange line, a pipe for sending at least one portion of the airboosted in the first booster to be cooled in the exchange line, a pipefor sending this cooled portion to the second booster at a secondintermediate temperature from the exchange line, a pipe for sending airfrom the second booster to the exchange line in order to be cooled, apipe for sending the cooled air coming from the second booster from theexchange line to a means for expanding and then into the system ofcolumns, with the exchange line being designed in such a way that thefirst temperature differs from the second temperature by at most 10° C.and the first and second temperatures being between −145° C. and −165°C., at least two turbines, a pipe for sending another portion of thehigh-pressure purified air, constituting possibly between 65% and 90% ofthe high-pressure purified air, from the exchange line to the twoturbines having one or more intake temperatures which is an intermediatetemperature or which are intermediate temperatures of the exchange line,pipes for sending air from the two turbines to the system of columns,with the first booster being coupled to one of the two turbines and thesecond booster to the other of the two turbines, a pipe for sendingliquid oxygen, pressurised at a pressure lower than or equal to 16 bars,more preferably between 10 and 16 bars, is vaporised in the exchangeline and an energy dissipating device coupled to at least one of theboosters.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood with regard to the followingdescription, claims, and accompanying drawings. It is to be noted,however, that the drawings illustrate only several embodiments of theinvention and are therefore not to be considered limiting of theinvention's scope as it can admit to other equally effectiveembodiments.

FIG. 1 shows a method according to an embodiment of the invention.

DETAILED DESCRIPTION

The invention shall be described in more detail by referring to thefigure which shows a method for separating air according to theinvention.

An airflow is compressed in a main compressor 3 to a pressure at least 3bars above the pressure of the column 31, which is the medium-pressurecolumn of a double column for separating air by means of cryogenicdistillation. The compressed air is purified in a purification unit 7 inorder to form the purified flow 9. The purified flow is sent to theexchange line 11 without having been cooled and in the exchange line itis cooled to a first intermediate temperature. At this temperature, theair is divided into a portion 13 and a portion 14. The portion 13 entersinto a single first single-stage booster 15 at the first intermediatetemperature wherein it is boosted. The boosted air is sent to theexchange line 11 wherein it is cooled again to a second intermediatetemperature, lower than the first intermediate temperature. At thissecond intermediate temperature, at least one portion of the air boostedin the booster 15, even all of the air 13, is boosted in a single secondsingle-stage booster 25.

The first intermediate temperature differs from the second temperatureby at most 10° C. and the first and second temperatures are between−145° C. and −165° C.

The first intermediate temperature can possibly be higher or equal tothe second intermediate temperature.

Each of the output temperatures of the boosters 15, 25 is between 110°C. and −150° C., more preferably between −125° C. and −145° C.

The doubly boosted flow 13 is sent to the exchange line at the pressurerequired for the vaporisation of a flow of pressurised oxygen. Theboosted flow 13 cooled to this pressure to the cold end of the exchangeline 11 and is condensed. At the output of the exchanger, the flow isexpanded, and is sent to the medium-pressure column 31.

The rest of the air 14 is divided into two or three portions. Accordingto an alternative, all of the air 14 is divided into two portions. Oneportion 19 is sent to a turbine 17 having an intake temperature which isa third intermediate temperature of the exchange line, then is sent ingaseous form to the medium-pressure column 31.

Another portion 21 is sent to a turbine 27 having an intake temperaturewhich is a fourth intermediate temperature of the exchange line, higherthan the third temperature, then is sent in gaseous form to themedium-pressure column 31. More preferably the portions 19, 21 are mixedin order to form a single flow 23.

Otherwise in addition to the portions 19, 21, a portion 26 of thehigh-pressure air can possibly continue to be cooled to the cold end ofthe exchange line 11 and is condensed. At the output of the exchanger,it will be expanded in a valve and sent to the system of columns, forexample to the medium-pressure column 31.

The double column comprises a medium-pressure column 31 and alow-pressure column 33, thermally connected together with reflux flows39, 41 in a known manner.

The low-pressure column 33 produces a flow of nitrogen 43 which isheated in the exchange line 11. It also produces liquid oxygen 35 in thetank which is pressurised at a pressure between 10 and 16 bars and isvaporised in the exchange line in order to form pressurised gaseousoxygen.

It can be considered to vaporise liquid oxygen at two differentpressures in this way or to vaporise liquid nitrogen or liquid argon,possibly pressurised at the same time as the liquid oxygen.

In the case where two products are vaporised in the exchange line (orone product at two different levels of pressure), a portion of the flow13 can continue to be cooled to the cold end of the exchanger and not beboosted by the booster 25. This fraction of flow will condense. At theexchanger output, it will be expanded in a valve and sent to the systemof columns, for example to the medium-pressure column 31.

The booster 15 is driven at least in part by one of the two turbines 17or 25, and the booster 25 by the other turbine 25 or 17. In each case,there can also be a motor or a generator coupled to the compressor. Anenergy dissipating device 22, 24, for example a valve, preferably an oilvalve system, will be integrated into at least one of the twoturbine/compressor systems 15/17, 25/27.

While the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives,modifications, and variations will be apparent to those skilled in theart in light of the foregoing description. Accordingly, it is intendedto embrace all such alternatives, modifications, and variations as fallwithin the spirit and broad scope of the appended claims. The presentinvention may suitably comprise, consist or consist essentially of theelements disclosed and may be practiced in the absence of an element notdisclosed. Furthermore, if there is language referring to order, such asfirst and second, it should be understood in an exemplary sense and notin a limiting sense. For example, it can be recognized by those skilledin the art that certain steps can be combined into a single step.

The singular forms “a”, “an” and “the” include plural referents, unlessthe context clearly dictates otherwise.

“Comprising” in a claim is an open transitional term which means thesubsequently identified claim elements are a nonexclusive listing (i.e.,anything else may be additionally included and remain within the scopeof “comprising”). “Comprising” as used herein may be replaced by themore limited transitional terms “consisting essentially of” and“consisting of” unless otherwise indicated herein.

“Providing” in a claim is defined to mean furnishing, supplying, makingavailable, or preparing something. The step may be performed by anyactor in the absence of express language in the claim to the contrary arange is expressed, it is to be understood that another embodiment isfrom the one.

Optional or optionally means that the subsequently described event orcircumstances may or may not occur. The description includes instanceswhere the event or circumstance occurs and instances where it does notoccur.

Ranges may be expressed herein as from about one particular value,and/or to about another particular value. When such particular valueand/or to the other particular value, along with all combinations withinsaid range.

All references identified herein are each hereby incorporated byreference into this application in their entireties, as well as for thespecific information for which each is cited.

1-15. (canceled)
 16. A method for separating air by means of cryogenicdistillation in an installation comprising a system of columns, of whichone column operating at the highest pressure called the medium pressurewherein: all of the air is brought to a high pressure, at least 3 barshigher than the medium pressure, purified at this pressure in apurification unit and air is sent at the output temperature from thepurification unit to an exchange line; all of the purified air is cooledin the exchange line and a portion constituting between 10% to 35% ofthe purified air is boosted by means of at least one first single-stagebooster (15) and sucking at a first intermediate temperature of theexchange line; at least one portion of the boosted air in the firstbooster is cooled in the exchange line, boosted by means of at least onesecond single-stage booster and sucking at a second intermediatetemperature of the exchange line and is sent back into the exchange linewhere it is cooled, then is liquefied, possibly at the cold end of theexchange line and is sent into the system of columns after expansion;another portion of the high-pressure purified air, constituting possiblybetween 65% and 90% of the high-pressure purified air, is cooled in theexchange line then at least partially expanded in at least two turbineshaving one or more intake temperatures which is an intermediatetemperature or which are intermediate temperatures of the exchange linethen sent to the system of columns in order to be separated; the workgenerated by the expansion of the air is used at least partially for thecryogenic compression carried out by the first and/or the second boosterby coupling the first booster to one of the two turbines and the secondbooster to the other of the two turbines; and liquid oxygen is vaporizedin the exchange line, wherein all of the air purified in thepurification unit is sent at the output temperature of the purificationunit to an exchange line, the liquid oxygen is pressurized at a pressurebelow or equal to 16 bars in order to be vaporized in the exchange line,an energy dissipating device is coupled to at least one of the boosters,the first temperature differs from the second temperature by at most 10°C. and the first and second temperatures are between −145° C. and −165°C.
 17. The method as claimed in claim 16, wherein the two turbines havedifferent intake temperatures, constituted by the third intermediatetemperature and a fourth intermediate temperature of the exchange line.18. The method as claimed in claim 17, wherein the third temperature islower than the first temperature.
 19. The method as claimed in claim 17,wherein the third temperature differs from the fourth temperature by atmost 20° C.
 20. The method as claimed in claim 17, wherein the thirdtemperature differs from the fourth temperature by at most 10° C. 21.The method as claimed in claim 16, wherein the first temperature ishigher than the second temperature.
 22. The method as claimed in claim16, wherein the first temperature is lower than or equal to the secondtemperature.
 23. The method as claimed in claim 16, wherein a portion ofthe energy generated by at least one of the turbines is dissipated. 24.The method as claimed in claim 23, wherein a portion of the energy isdissipated by means of an oil valve system connected to the turbine. 25.The method as claimed in claim 16, wherein all of the air boosted in thefirst booster is sent to the second booster.
 26. The method as claimedin claim 16, wherein all of the air purified in the purification unit issent to the exchange line at the output pressure of the purificationunit.
 27. The method as claimed in claim 16, wherein the systemcomprises a double column for separating air comprising a first columnand a second column operating at a lower pressure than the first andwherein the air expanded in the two turbines is sent to the firstcolumn.
 28. The method as claimed in claim 16, wherein all of the airintended for the separating is sent to the hot end of the exchange line.29. The method as claimed in claim 16, wherein the first temperature iscolder than the output temperature of the second booster.
 30. The methodas claimed in claim 16, wherein the output temperature or temperaturesof the first and/or of the second booster is/are between −110° C. and−150° C.
 31. The method as claimed in claim 16, wherein the outputtemperature or temperatures of the first and/or of the second boosteris/are between −125° C. and −145° C.
 32. The method as claimed in claim16, wherein the liquid oxygen is pressurised at a pressure between 10and 16 bars.